The various random devices produce high quality
random output data. Entropy data is collected from system activity (like disk,
network, and clock device interrupts and such), and then run through various
hash or message digest functions to generate the output. All the random
devices are expected to provide high quality pseudo-random output data.

The arc4random(3)
function in userland libraries seeds itself from this device (or with the
sysctl(3)
interface), providing a second level of ARC4 hashed data without the need to
access these devices every time.